This is how I would book the main event for Wrestlemania:
Brock and Reigns have a slobber-knocker fight neither one able to put the other way (shades of Wrestlemania 6 Hogan vs. Warrior)
Ref gets knocked out and while this happens Heyman gets involved trying to help Brock. Paul hits Reigns over the head with the belt as Brock is getting to his feet. This sends Brock into a fit of rage on Heyman as he wanted to win cleanly. Brock reluctantly covers Reigns and he retains. While he is celebrating in the ring Heyman convinces Reigns that it was Brock who attacked him. This is where the double turn happens. Reigns hits the ring with weapons and goes off on Brock. Absolute beat down by the new heel Reigns on the new face champ Brock. Heyman encourages Reigns as he has jumped ship to Reigns side. While Brock is laying in the middle of the ring Reigns motions towards the ramp and Rollins music hits. He cashes in his contract and is crowned the new champ. He stands victorious in the ring with his new partners Reigns and Heyman.
Brock now as a face has a redemption story line moving forward. Revenge on Paul, Roman and getting the title back.
Heyman pushes his new heel group with 2/3rds of the old shield.
WWE should hire me